Ink-jet inks typically comprise an ink vehicle and a colorant, the latter of which may be a dye or a pigment. Dye-based ink-jet inks used in photographic image printing are almost always water-soluble dyes. As a result, such dye-based ink-jet inks are usually not very water fast, i.e. images tend to shift in hue and edge sharpness is reduced upon exposure to humid conditions. In addition, images created from these water-soluble dye-based ink-jet inks tend to fade over time, such as when exposed to ambient light and/or air. Pigment-based inks on the other hand, allow the creation of images that are vastly improved in humid fastness and image fade resistance. Pigment based images, however, are inferior to dye-based ink-jet inks with respect to the desirable trait of color saturation.
Print media surfaces play a key role in the overall quality of ink-jet produced printed images. Papers used for ink-jet printing have typically included high-quality or wood-free papers designed to have high ink absorptivity. These papers are functionally good for ink-jet printing because the ink-jet inks may be absorbed readily and dry quickly. However, such papers often do not allow for a crisp or sharp image. In order to attain enhanced print quality and image quality as in a photograph, special media has been developed to work with aqueous inks, which can be separated into two broad groups: porous media and swellable media.
With porous media, an ink receiving layer can comprise porous metal oxide or semi-metal oxide particulates (usually silica or alumina) bound together by some polymer binder, and optionally, mordants or ionic binding species, e.g., cationic binding species for use with anionic dyes or anionic binding species for use with cationic dyes. During printing, ink is quickly adsorbed onto the surface which is porous in nature, and if an ionic binding species is present, the colorant can be attracted to the ionic species of opposite charge. This type of media has the advantage of relatively short dry-times, good smearfastness, and often, acceptable water and humidity resistance. Conversely, with swellable media, an ink receiving layer is present that comprises a continuous layer of a swellable polymer that is not physically porous. Upon printing, ink is absorbed as water contacts and swells a polymer matrix of the coating. The colorant, which is typically a dye, can be immobilized inside the continuous layer of the polymer with significantly limited exposure to the outside environment. Advantages of this approach include much better fade resistance (in both light and dark conditions) than is present with porous media. However, swellable media requires a longer dry time, is not typically as crisp in image quality, and exhibits poor smearfastness.
Though both swellable media and porous media each provide unique advantages in the area of ink-jet printing, due to the image crispness and fast dry time achievable from porous media, there is some trending in the direction of the use of porous media. However, the preparation of porous media has unique challenges. For example, many porous media formulations tend to crack upon coating on a media substrate and drying, and further, with certain coatings, a hazy appearance of inks printed thereon can occur.